On the 13th of December 2019, Grégory Guyot will defend his PhD entitled "contribution to the characterisation of air entrainment in hydroelectric power plants"
This PhD project was supervised by Alain Cartellier (LEGI) and Jean-Philippe Matas (LMFA, Lyon) and carried out in collaboration with EDF.
The defense will take place at 2:30 pm in the conference room of the LEGI (K118).
We study the entrainment of air created by large-dipped jets. Direct applications in the field of hydroelectricity are energy optimization of plants, environmental management around infrastructures and prediction of extreme flood impacts downstream of dams. For such applications, it is necessary to quantify the penetration depth of the bubble cloud, predict the distribution of bubbles and the amount of air entrained. Predictive models describing these three points would be key engineering tools. The literature review shows that many models, particularly those applied to penetration depth or entrained air flow, exist but are limited to a narrow range of parameters. A first experiment was carried out on circular jets with a diameter of about ten centimetres and a drop length of 2.6 m. Comparison of the results with the existing models reveals that these models do not correctly capture the penetration depth, the size of the generated bubbles or the entrained air flow rate. This experiment shows that the jets flap and have an aerated structure at impact, with significant corrugations. In a second smaller scale experiment, we have characterized the influence of the jet movement on the penetration depth. It appears that the oscillation of the jet can lead to a decrease of the penetration depth, but this influence is not sufficient to explain the differences between experiments and models in the large scale experiment. We then designed and operated a third experiment to analyze jets with nozzle speeds between 2 and 30 m/s, nozzle diameters between 24 and 213 mm for a drop height of about 10 m. It is also possible to investigate bubble clouds with this experiment, since the jets are received in a 5 m diameter pit with a depth of up to 23 m. The cross-comparison of measurements made by analyzing high-frequency videos of jet drop, force at impact and optical probe measurements in bubble clouds evidence that the state of the jet at impact has a significant influence on air entrainment. We clearly identify jets with different shapes and air flows than those previously studied. Our analysis also allows us to propose a simple model without any adjustment parameters, based on a balance of forces applied to the bubble cloud. This model predicts the penetration depth of the bubble cloud for a wide range of jet scales.